Recently, mutations in both the adenomatous polyposis coli (APC) tumor suppressor and beta-catenin, the vertebrate Armadillo orthologue, have been implicated in the onset of both colorectal neoplasia and melanoma in humans. Cells harboring these mutations exhibit increased nuclear levels of beta-catenin, and are presumably transformed by constitutive activation of the Wnt/Wingless (Wg) signaling pathway. Because the Wnt/Wg signaling pathway has been conserved between distantly related organisms, we propose to utilize the fruit fly, Drosophila melanogaster, as a model system for identifying novel genes whose activity regulates the function of Armadillo. Arm is a rate-limiting factor required for Wg signal transduction and the maternally contributed stores of Armadillo fail to provide the requisite amount of cytoplasmic Armadillo protein required for the patterning of the ventral larval cuticle, as all is recruited into the adherens junction. A screen for haplo-insufficient modifiers was initiated to identify new genes and/or pathways whose activity antagonizes Armadillo function. Utilizing chromosomal deficiencies, approximately 65% of the Drosophila genome has been screened for suppressors. To date, 13 independent interacting regions have been identified on chromosomes 2, 3 and 4 and single genes (puc, hid, and dTCF) identified in three of these intervals. The specific aims of this proposal are to: (i) define the genetic and/or biochemical interactions between puckered (puc), a dual-specificity phosphatase identified in the original screen, and Armadillo, and (ii) identify and characterize additional genes, in the remaining intervals, that modify Armadillo. Because of the high degree of conservation of the Wnt(Wg signaling system in distantly related organisms, the role various molecules defined in this study can then be re-examined in systems where molecular genetic techniques are not as developed, but where models of disease states reflect the onset and progression of clinical disorders. Together, such an approach should provide new insight into the etiological potential of Arm/beta-catenin-dependent signaling.